The present invention relates to methods for making acrylic-functional silicone compositions. More particularly, the present invention relates to methods for making acrylic-functional silanes and siloxanes which employ novel inhibitors to prevent thermal free radical polymerization of the acrylic-functional reactants.
Ultraviolet light curable silicone compositions have gained widespread acceptance in many fields, for instance as release compositions for use with pressure sensitive adhesives. One reason for this success is that ultraviolet light curable compositions overcome many of the disadvantages of solvent-based systems such as the need for energy intensive ovens and solvent-recovery apparatus.
A variety of methods for preparing ultraviolet light curable silicone compositions are disclosed in the prior art.
Ohto et al., U.S. Pat. No. 3,865,588, teaches the addition reaction between (i) a compound having an unsaturated radical represented by the general formula ##STR1## where R.sup.1 is a hydrogen atom, phenyl radical, or a halogen substituted phenyl radial, and R.sup.2 is a hydrogen atom or a methyl radical and also having an aliphatic unsaturated bond, and (ii) a silane represented by the general formula ##STR2## where R.sup.4 is a monovalent hydrocarbon radical or a halogen substituted monovalent hydrocarbon radical having from 1 to 10 carbon atoms, Z is a halogen atom, an acetoxy radical, a hydroxyl radical or an alkoxy radical having from 1 to 4 carbon atoms, and a is number representing 0 or 1, in the presence of a catalyst such as chloroplatinic acid. Ohto et al. also teach that the temperature of the reaction system can be raised in order to accelerate the reaction, however, in such case it is preferable to add some thermal polymerization inhibitor such as quinones, e.g., hydroquinone or benzoquinone, amine salts or hydrazine salts. Reference 8 of Ohto et al. discloses that 115 parts of methylhydrogendichlorosilane, 149 parts of monomethyltrichlorosilane, 211 parts of monophenyltrichlorosilane and 516 parts of dimethyldichlorosilane were added dropwise to a mixture of 1000 parts of toluene, 100 parts of methanol, and 5000 parts of water, which mixture was kept stirring at 5.degree.-10.degree. C. The reaction system was then washed until it's pH became 7.0, and then toluene was distilled from the system until the siloxane concentration was 50 percent. Subsequently a mixture of 138 parts of allyl methacrylate, 0.2 part of a 2 percent isopropanol solution of chloroplatinic acid and 0.5 part hydroquinone was added. When the addition was over the mixture was heated for ten hours at which time there was obtained a polymerizable silicone compound having a pour point of 48.degree. C. Reference 8 thus illustrates that it is necessary to hydrolyze and alkoxylate the halosilane before effecting acrylation.
Martin, U.S. Pat. No. 3,878,263, discloses that acrylate-functional silanes and siloxanes may be prepared by the addition of a compound of the formula ##STR3## where R is a hydrogen atom or a C.sub.1-12 monovalent hydrocarbon radical, and G is an unsaturated radical such as vinyl, allyl, methallyl, or butenyl, with a compound of the formula ##STR4## where R.sup.1 is a hydrogen atom or a monovalent hydrocarbon radical, e is a number from 0 to 2, inclusive, and Z is selected from the class consisting of R.sup.1, OR.sup.1, and OSi(R.sup.2).sub.3, where R.sup.2 is a monovalent hydrocarbon radical, halogenated monovalent hydrocarbon radical or a cyanoalkyl radical. According to Martin, the reaction preferably is carried out in the presence of a polymerization inhibitor for acrylic acid or methacrylic acid, such as hydroquinone or N,N'-diphenylphenylene diamine. Again the artisan is taught that only alkoxy-functional silanes can be acrylated.
Tanaka, U.S. Pat. No. 4,139,548, discloses the preparation of methyldi(trimethylsiloxy)silylpropylglycerolmethacrylate by reacting methyldi(trimethylsiloxy)silylpropyl(oxypropylene oxide) with methacrylic acid in the presence of a catalyst. In order to prevent the polymerization of methacrylic acid, it is desirable to carry out the reaction in the presence of a polymerization inhibitor such as hydroquinone, hydroquinone monomethyl ether or sulfur.
Cully et al., U.S. Pat. No. 4,201,808, discloses that radiation curable compositions having acrylic functionality can be stabilized against premature polymerization during storage by the addition of a conventional polymerization inhibitor such as hydroquinone, monomethyl ether of hydroquinone, phenothiazine, di-t-butyl paracresol, etc. in concentrations on the order of 0.1 weight percent or less.
Careful consideration of the foregoing disclosures reveals that in each instance the polymerization inhibitor was employed in a system free of chloride.
The present applicant determined that it would be desirable to effect acrylation of chlorosilanes, thus eliminating, the need to first alkoxylate the silane before reacting with an acrylate-containing compound, for example, as described in Ohto et al, U.S. Pat. No. 3,865.588, or Martin, U.S. Pat. No. 3,878,263. Accordingly, there was prepared a reaction mass of ##STR5## and an amount of platinum catalyst effective for promoting the reaction of these compounds. In order to prevent the thermal free radical polymerization of allylmethacrylate there was added a hydroquinone inhibitor. It was found that hydroquinone compounds are not effective for inhibiting the thermal free radical polymerization of the organic acrylate in the presence of chloride-containing compounds such as halosilanes.